1. Field of the Invention
The invention relates generally to methods for growing carbon nanotubes and, particularly, to a laser-based method for growing an array of carbon nanotubes.
2. Discussion of Related Art
Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs are electrically conductive along their length, chemically stable, and capable, individually, of having a very small diameter (much less than 100 nanometers) and large aspect ratios (length/diameter). Due to these and other properties, it has been suggested that CNTs can play an important role in various fields, such as microscopic electronics, field emission devices, thermal interface materials, etc.
Generally, there are three conventional methods for manufacturing CNTs. The first method is the arc discharge method, which was first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). The second method is the laser ablation method, which was reported in an article, by T. W. Ebbesen et al., entitled “Large-scale Synthesis of Carbon Nanotubes” (Nature, Vol. 358, 1992, pp. 220). The third method is the chemical vapor deposition (CVD) method, which was reported in an article by W. Z. Li, entitled “Large-scale Synthesis of Aligned Carbon Nanotubes” (Science, Vol. 274, 1996, pp. 1701). The CVD method is advantageously useful in synthesis of an array of carbon nanotubes and is beneficial in mass production, improved length controllability, and compatibility with conventional integrated circuit process, etc.
Generally, three main CVD methods, i.e. thermal CVD, plasma-enhanced CVD, and laser-induced CVD, have been developed for the synthesis of arrays of carbon nanotubes. In conventional laser-induced CVD method, an opaque substrate, such as silicon, is disposed with a catalyst in a closed reactor filled with reactant gases, and, typically, either an argon ion laser or CO2 laser is employed to directly irradiate laser light on the substrate to heat the substrate to a reaction temperature. By locally laser-heating the substrate, carbon nanotubes can be synthesized on the substrate.
However, the above-described laser-assisted CVD method is performed in a closed reactor filled with reactant gases. Thus, the above-described method has required a complicated reaction device, and it is difficult to build and/or maintain a large-scale reactor device for CVD growth of carbon nanotubes on a large area substrate. Moreover, the newly grown carbon nanotubes tend to be directly exposed to an intense laser field, which can potentially damage the newly grown carbon nanotubes. Thus, the morphology of the obtained carbon nanotubes can be compromised. These carbon nanotubes typically do not sufficiently exhibit the useful properties desired in an array of carbon nanotubes.
What is needed, therefore, is a new laser-induced method for growing an array of carbon nanotubes that does not require a closed reactor system and that can avoid damaging the newly grown CNTs with an intense laser.